The invention relates to a signal analizer using charge transfer devices, and in particular to a compensator for charge transfer inefficiency distortion in charge coupled device convolvers.
Signal or wave analysis has been carried out using various types of analytical circuits. One of these types of circuits involves a mathematical process known as convolving. A general discussion of this process will be found in the third chapter of a book by Oppenheim and Willsky, entitled "Signals and System", Prentice Hall, 1983. The implementation of this process, as carried out in the present invention, involves sampling a plurality of discreet portions of a signal or wave on a real-time delayed basis and producing therefrom a summation signal having the desired characteristics which accurately may represent the input signal or wave. The apparatus for doing this is referred to herein as a convolver. The specific convolver circuitry used herein includes charge coupled devices connected in a special network, and it is to this type of circuitry that the present invention is directed.
Charge tranfer devices or charge coupled devices (CCD) comprise a series of similar semiconductor cells each having an input, an output, a capacitance for the storage of charge which represents information, and at least one control electrode for charge transfer timing control. The input of substantially each cell is coupled to the output of the preceding cell. A control means provides clock signals to control electrodes of each of the cells in a manner to control the charge transfer from one cell to the next.
Charge transfer devices similar to that just described are known and disclosed in U.S. Pat. No. 3,666,972. These devices have the drawback that during transfer of a signal charge packet or signal sample from one cell of the charge transfer device to a following cell, a residual charge remains in the preceding cell such that the magnitude of the charge packet transfer is less than the preceeding cell. As a consequence, the residual charge is added to a following signal charge packet which passes through the series of cells. The description of this problem can be found in the report of the "Third International Conference on the Technology and Application of Charge Coupled Devices" held in September, 1976 at Edingburgh, in an article by Chowaniec and Hobson on pages 227-231 entitled "An Analysis of CCD Recursive Filters With Application to MTI Radar Filters."
A charge transfer inefficiency compensator to overcome the problem described above is shown in U.S. Pat. No. 4,455,666 issued to Arthur H. M. van Roermund. In the last referred to patent, a detector is coupled to the output of one of the cells for detecting the magnitude of a signal charge packet on the output thereof. A compensation charge packet is derived from the selected cell and is fed back to a preceding cell but which lags the charge packet by one sample period.
In the specific application of a charge coupled device used in a convolver, the invention of the latter mentioned patent will not provide the necessary signal connection. This is so since in the convolver application, each of the cells in a series of cells is relatively weighted to provide the convolver output.